There has been a growing interest in the manufacture and use of microfluidic systems for the acquisition of chemical and biological information. In particular, when conducted in microfluidic volumes, complicated biochemical reactions may be carried out using very small volumes of liquid. Among other benefits, microfluidic systems improve the response time of reactions, minimize sample volume, and lower reagent consumption. When volatile or hazardous materials are used or generated, performing reactions in microfluidic volumes also enhances safety and reduces disposal quantities.
Traditionally, microfluidic devices have been constructed in a planar fashion using techniques that are borrowed from the silicon fabrication industry. Representative systems are described, for example, in some early work by Manz et al. (Trends in Anal. Chem. (1990) 10(5): 144-149; Advances in Chromatography (1993) 33: 1-66). In these publications, microfluidic devices are constructed by using photolithography to define channels on silicon or glass substrates and etching techniques to remove material from the substrate to form the channels. A cover plate is bonded to the top of the device to provide closure. Miniature pumps and valves can also be constructed to be integral (e.g., within) such devices. Alternatively, separate or off-line pumping mechanisms are contemplated.
More recently, a number of methods have been developed that allow microfluidic devices to be constructed from plastic, silicone or other polymeric materials. In one such method, a negative mold is first constructed, and plastic or silicone is then poured into or over the mold. The mold can be constructed using a silicon wafer (see, e.g., Duffy et al., Analytical Chemistry (1998) 70: 4974-4984; McCormick et al., Analytical Chemistry (1997) 69: 2626-2630), or by building a traditional injection molding cavity for plastic devices. Some molding facilities have developed techniques to construct extremely small molds. Components constructed using a LIGA technique have been developed at the Karolsruhe Nuclear Research center in Germany (see, e.g., Schomburg et al., Journal of Micromechanical Microengineering (1994) 4: 186-191), and commercialized by MicroParts (Dortmund, Germany). Jenoptik (Jena, Germany) also uses LIGA and a hot-embossing technique. Imprinting methods in PMMA have also been demonstrated (see, Martynova et al, Analytical Chemistry (1997) 69: 4783-4789) However, these techniques do not lend themselves to rapid prototyping and manufacturing flexibility. Additionally, the foregoing references teach only the preparation of planar microfluidic structures. Moreover, the tool-up costs for both of these techniques are quite high and can be cost-prohibitive.
Various conventional tools and combinations of tools are used for separations and detections when performing analyses in conventional macroscopic volumes. Such tools include, for example: filters, metering devices, columns, valves, sample injectors, heaters, coolers, mixers, splitters, diverters, and electrodes (such as are used to induce electrokinetic flow and to perform electrophoretic separations). Attempts to conduct separations or detections in microfluidic volumes have been stifled by difficulties such as making such tools in microfluidic scale and then integrating such tools into microfluidic devices. Another difficulty is accurately measuring stoichiometric microfluidic volumes of reagents and solvents to perform analyses on a microfluidic scale. Additionally, difficulties in rapidly prototyping microfluidic devices are compounded by attempts to incorporate multiple analytical tools.
A particular challenge that has arisen in the design and fabrication of microfluidic devices is the proliferation of inputs and outputs associated with such devices. For example, PCT Patent Application WO 99/19717, entitled “Laminate Microstructure Device and Methods for Making Same,” by Aclara Biosciences, Inc. (the “Aclara Application”) discloses a microfluidic device, which includes multiple microfluidic structures therein. FIG. 1 illustrates a device 100 similar to that disclosed in the Aclara Application. The device 100 includes eight microfluidic structures 102A-102H. Each of the microfluidic structures 102A-102H has eleven input/output (“I/O”) ports 103A-103N. Consequently, operation of the device 100 would require eighty-eight I/O connections. Furthermore, it is anticipated that microfluidic devices may include substantially more than eight microfluidic structures per device. Thus, the number of I/O connections for a more feature-dense device could be significantly higher than eighty-eight.
One benefit of microfluidic devices is the ability to perform multiple experiments in a small area. The large number of I/O connections required by the device 100 would have a tendency to either expand the size of the device to accommodate the connections or complicate fabrication and operation of the device. In particular, providing a large number of I/O connections in a compact area elevates the likelihood of fabrication and/or operational errors.
The microfluidic devices described herein may include any number of parallel functional features and related inputs and outputs. Although the prior art and illustrative embodiments of the invention shown herein each have a particular number of such features, such features are numbered and lettered to reflect the fact that additional such features may be included. For example, in FIG. 2A, the functional features are numbered 106A-106N, where “N” represents the total number of such features included in the device 104. Whereas in the illustrated device 104 the “N” represents the third such functional feature, such a device 104 could include tens, hundreds or even more of such functional features providing the desired functionality according to the invention.
FIGS. 2A-2E use simplified block diagrams to illustrate various permutations of desirable microfluidic devices and difficulties created by the need for multiple I/O connections. FIG. 2A is a simplified representation of a device 104 similar to that shown in FIG. 1. The device 104 includes a plurality of functional features 106A-106N. A functional feature can be any structure for performing a desired fluidic operation, including, but not limited to one or more mixers, reactors, separation chambers, and any combinations thereof. Each functional feature 106A-106N has a sample input 108A-108N and an output 108A′-108N′. In addition, each functional feature 106A-106N may have a plurality of reagent inputs 110A-110N, 112A-112N, 114A-114N. For simplicity, the device 104 is shown with only one sample input, one output, and three reagent inputs for each functional feature 106A-106N; however, any number of inputs and outputs for samples and reagents may be used as necessitated by the desired fluidic function to be performed by the functional feature. Accordingly, the number of I/O connections required by such a device 104 equals the number of I/O connections per functional feature multiplied by the number of functional features.
If the functional features 106A-106N perform substantially identical operations in parallel, then it is likely that the same set of reagents will be used in each of the functional features. If the device 104 is used to perform parallel operations using the same reagents on a variety of samples, then the number of I/O connections may be reduced if inputs for reagents common to more than one functional feature are combined, as shown in FIG. 2B. A device 120 includes a plurality of functional features 122A-122N. Each functional feature 122A-122N has a sample input 124A-124N and an output 124A′-124N′. Two common reagent inputs 126, 128 provide reagents to the functional features 122A-122N. Because reagent can be provided to all functional features 122A-122N from the two inputs 126, 128, the total numbers of I/O connections for reagents can be reduced by (N−1)×Y, where “N” is the number of functional features and “Y” is the number of reagent inputs per functional feature. Thus, if the device 120 includes eight functional features with two reagent inputs for each, this approach would result in only two common reagent inputs, rather than the sixteen independent reagent inputs that would be required for a device such as that shown in FIG. 2A.
The techniques used to fabricate microfluidic devices typically rely on machining or etching the surface of a planar material to produce the desired microfluidic structure. As a result, these microfluidic structures typically are provided in a single plane. One consequence of this approach is that it becomes difficult, if not impossible, to substantially expand the functionality and complexity of the fluidic operations due to structural limitations. For example, as shown in FIG. 2B, the addition of a third common reagent input 130 (shown in ghosted lines) and channels 132, 134 to carry reagent from the input 130 to the functional features 122A-122N results in the intersection (or “channel crossing”) of these channels 132, 134 with other reagent channels 136, 138 at intersection points 140, 142. Because these structures all are defined in a single plane, the channel crossings 132, 134, 136, 138 will result in unintended combining of the reagents, essentially rendering the device 120 inoperable for most scientific purposes. Likewise, as shown in FIG. 2C, a plurality of reagent inputs 150A-150N may be used to provide reagents to two functional features 152A, 152B in a device 149. If, however, further functional features 152N are added, channels 154A-154N from any reagent input 150A-150N in excess of two will result in problematic channel crossings 156A-156N.
Thus, in a two dimensional device, it is impossible to use of more than two common non-intersecting reagent inputs when more than two functional features are used. Likewise, the use of more than two functional features is impossible when more than two common non-intersecting inputs are used. Of course, it may be possible to use small hoses to allow crossing lines to “jump over” the intersection. However, such an approach would substantially increase the manufacturing complexity of such a microfluidic device as well as compound the likelihood of component failures that could render the device inoperable.
The use of common inputs, while potentially simplifying the I/O connections to a microfluidic device, also may create additional problems. As a result of the very small dimensions of microfluidic structures, fluids moving through such structures are characterized by very low Reynolds Numbers (corresponding to laminar flow) and flow dynamics that are heavily affected, if not dominated, by surface interactions. Thus, fluids in microfluidic structures often exhibit surprising and unexpected properties. For example, when fluid traveling through a microfluidic structure encounters a split or fork in a channel, the fluid may flow through only one fork or only the other—not dividing and distributing evenly between the two, as would be expected in conventional macrofluidic systems. Alternatively, the flow may split, but not evenly. As a consequence of this behavior, it may be difficult to consistently and accurately divide and distribute a reagent stream to a plurality of functional features, simply because it may be difficult to predict the particular flow paths that will be adopted by a given fluid flowing within a multi-path microfluidic structure.
It has been observed that fluid flow behavior within microfluidic structures may be influenced by the fluidic impedance encountered by the fluid. The presence and magnitude of fluidic impedance depends on a number of factors, such as interaction between the fluid and the surface of the structure (“surface interactions”); the pressure driving the fluid (“fluid pressure”); the pressure resisting fluid flow (“backpressure”); the physical arrangement of the microfluidic structure (“structural geometry”); and the characteristics of the fluid, including, but not limited to, mass, density, and viscosity (“fluid properties”). In particular, it has been noted that fluids divided and distributed from a single source or inlet (which may be a port, aperture or channel) into a plurality of branch channels tend to split evenly among the branch channels only when the impedance encountered by the fluid is substantially the same across all of the branch channels into which the fluid is being divided.
Thus, if a common input is used to divide and distribute a fluid among multiple functional features, care must be taken to match the impedance of each channel carrying reagent from the common input to each of the functional features. For example, FIG. 2D illustrates a simple microfluidic device 170 having two functional features 172A-172B. Each of the functional features 172A-172B has a sample input 174A-174B and an output 176A-176B. Two common reagent inputs 178A, 178B provide reagent to the functional features via reagent channels 180A-180D. In this simple configuration, impedance matching among channels 180A-180D is provided by positioning the reagent inputs 178A-178B equidistantly from the functional features 172A-172B, thereby matching the length of each of the channels 180A-180D to each other. So long as such a simple arrangement is possible, this approach may provide the desired results. However, as a design becomes more complex, due to, for example, increased feature density or input positions required to maintain compatibility to a particular laboratory device, such careful positioning of reagent inputs may not be possible. Thus, as shown in FIG. 2E, in a device 190, having two functional features 191A-191B and common reagent inputs 192A-192B, it may be necessary to provide convoluted reagent channels 194A-194D. The convolutions of the reagent channels 194A-194D allow the channels 194A-194D to be the same length (therefore having substantially the same impedance, assuming that the other channel characteristics are constant) even though the reagent inputs 192A-192B are not equidistant from each of the functional features 191A-191B.
If the feature density increases substantially, however, the convolutions required to provide the desired impedance matching may become very complex, thereby complicating the design, fabrication, operation and validation of the device. Furthermore, any such device remains constrained by the channel intersection problem described above.
In addition, the vast array of microfluidic tools and designs available today and anticipated in the future can present an infinite number of I/O interface configurations. For example, in each of the examples described above it can be seen that the pattern of inputs and outputs for samples and reagents differs substantially from device to device. Moreover, in order to maintain impedance matching among common inputs and/or to avoid undesirable channel intersections, the actual positioning of these inputs and outputs may be driven by the function of the device rather than the interface of existing laboratory tools. Thus, connection of highly parallel microfluidic devices to existing tools may require customized interfaces and/or complexes of flexible tubing to allow connection to other devices and/or laboratory tools and instruments. Such interface requirements tend to enlarge the footprint of the device, complicate operation, complicate manufacture of the device and/or increase the complexity of other devices used in conjunction with the device.
Thus, it would be desirable to provide microfluidic devices with minimal numbers of I/O connections. It also would be desirable to provide microfluidic devices that accurately and reliably divide and distribute fluidic inputs to the various structures within the device. It also would be desirable to provide microfluidic devices that readily interface with existing laboratory tools.